I had recently did my first virgin softmod on Wii. I might blog about that in details later, but one thing I came across which I think is worth sharing is the problem of black screen on loading games from HDD using USB Loader GX.
I had checked many forums and also nothing really work. But I managed to solve it. So here it is. Before you go further, ensure the following:
1. Your Wii is properly softmodded. All steps works according to whichever guide you followed.
2. Your HDD is using WBFS file system.
I know many will disagree with point 2. But that exactly what happened to me. I was told NTFS works. So I copied a few games onto my NTFS drive using Wii Manager. Black screen. What I eventually did was to format it as WBFS. Load games using the WBFS Manager 3.01. Tada! Works like a charm.
I hope this is useful for you.